1) Field of the Invention
This invention pertains to the field of communications and, more particularly, to a method and apparatus for detecting correlation peaks.
2) Background of the Related Art
One problem that commonly arises in communication systems is achieving synchronization between a receiver and a transmitter. While a synchronization signal may be employed, it is not always practical to provide a separate synchronization signal from the transmitter to the receiver when the receiver is located remotely from the transmitter. Synchronization can be especially difficult in mobile communication systems where communications occur sporadically, producing timing drift between transmitter and receiver circuits and, consequently, requiring frequent resynchronization.
A well known and popular synchronization technique involves the use of a predetermined correlation sequence. In such a technique, a transmitter transmits a predetermined correlation sequence at a regular or predetermined interval--e.g. at the start of each transmission, during the first transmission of a communication session, or according to some other convenient arrangement. A communication receiver likewise is provided with the predetermined correlation sequence. The receiver is provided with a correlator which compares the received signal with the predetermined correlation sequence to determine when the predetermined correlation sequence has been received.
Whenever the correlator matches the received signal to the predetermined correlation sequence, it produces a signal indicating that a correlation event has occurred. The timing of the correlation event may be used by the communication receiver to synchronize an internal clock or timer with the received signal. Synchronization in this manner by means of a predetermined correlation sequence is widely employed in digital communication systems.
Correlators also may be used in communication systems employing spread spectrum communication techniques. One well known spread spectrum communication technique is direct sequence spreading. In a direct sequence spread spectrum communication system, a digital symbol is encoded with a predetermined spreading code sequence prior to transmission over a communication channel. By using a spreading code sequence operating at a rate that is a multiple of the digital symbol rate, the resulting spread spectrum signal has a chip rate which is several times the digital symbol rate. A spread spectrum receiver may be provided with a spreading code sequence correlator which despreads the received spread spectrum signal using the predetermined spreading code sequence in order to recover the original digital symbol.
In a spread spectrum system, during each symbol period the transmitter may transmit one of several spreading code sequences each corresponding to a different symbol. In such a case, the receiver must simultaneously search for a correlation event corresponding to each symbol. This may be accomplished by the use of several correlators, each matched to a different unique spreading code sequence.
A spreading code sequence correlator may be used to provide timing signals for a spread spectrum receiver. The correlator may continuously compare the received signal with the predetermined spreading code sequence to determine when a correlation event occurs. As the correlator receives a signal containing the predetermined spreading code sequence, the correlator output signal steadily builds to produce a correlation peak at the end of the predetermined spreading code sequence signifying a correlation event.
The timing of the correlation peak may be used by the receiver for synchronization. For example, the receiver may have apriori knowledge of the symbol period, T, required for each transmission of a spreading code sequence. T is also the expected time interval between successive correlation events. Thus, the receiver may produce a clock signal operating at the received signal's symbol rate. However, the clock signal must still somehow be time or phase synchronized with the timing of the received symbols and this may be accomplished by means of a correlator as described above.
However, the above synchronization techniques face certain obstacles, particularly when the communication channel is subject to noise, interference, fading, and other losses. The correlator may falsely signal a correlation event in response to noise or random bit patterns. Also, true correlation events may be masked whenever noise-induced chip errors reduce the level of the correlation peak. Moreover, multipath propagation conditions may result in the occurrence of several localized correlation "peaks" instead of a steadily increasing buildup to a single correlation peak at the end of the correlation sequence. These problems are especially prevalent during the initiation of a communication session when the receiver has no knowledge of the time when a correlation event should be expected.
Accordingly, it would be advantageous to provide an improved correlation peak detector incorporating a peak threshold signal which dynamically adjusts to match the level of a received signal. It would further be advantageous to provide a correlation peak detector which mitigates the effects of noise or spurious inputs on detection of correlation peaks. It would be further advantageous to provide a correlation peak detector output signal which may be used to synchronize receiver timing circuits with a received signal.